There have been employed N,N,N′,N′-tetra(aryl)-1,1′-biphenyl-4,4′-diamines as a charge transport material, as disclosed in, for example, JP-B No. 63-6864 (hereinafter, the term JP-B refers to Japanese Patent Publication). However, structurally symmetrical N,N′-bis-(4-methylphenyl)-N,N′-bisphenyl-1,1′-biphenyl-4,4′-diamine, which is superior in electric characteristics such as sensitivity, exhibits high crystallinity and crystal deposition during preparation of the photosensitive layer, rendering difficult the use thereof. In practice, there have been mainly used N,N′-bis-(3-methylphenyl)-N,N′-bisphenyl-1,1′-biphenyl-4,4′-diamine, as disclosed in, for example, JP-A Nos. 2001-356500 and 2002-40687, or a mixture containing it as a main component.
Since such structurally symmetrical N,N′-bis-(4-methylphenyl)-N,N′-bisphenyl-1,1′-biphenyl-4,4′-diamine is superior in electric characteristics such as sensitivity, there was attempted preparation of a charge transport layer not exhibiting the foregoing defect but it was difficult to prevent crystal deposition during preparation of the photosensitive layer.